When was the last time you saw a documentary that fundamentally changed the way you think? It’s no secret that just about every political and socially-minded documentary shown at Sundance is preaching to the liberal-left choir. The issue may be dairy farming, human rights abuses in Vladimir Putin’s Russia, the marketing of AIDS drugs, or Occupy Wall Street (to list the topics of four festival docs this year), but the point of view is almost always conventionally “progressive” and orthodox. So when Robert Stone, who may be the most under-celebrated great documentary filmmaker in America (watch Oswald’s Ghost if you want to touch the elusive truth of the JFK assassination), arrived at Sundance this year with Pandora’s Promise, a look at the myths and realities of nuclear power, he was walking into the lion’s den. For this isn’t a movie that preaches to the choir. It’s a movie that says: “Stop thinking what you’ve been thinking, because if you don’t, you’re going to collude in wrecking the world.”Pandora’s Promise is built around what should be the real liberal agenda: looking at an issue not with orthodoxy, but with open eyes.

In Pandora’s Promise, Stone interviews a major swath of environmentalists, scientists, and energy planners, all of whom spent years being anti-nuclear power — and then, as they began to look at the evidence, changed their minds. The film begins with a deep examination of the psychology of the anti-nuclear view: how it took hold and became dogma. It goes all the way back to 1945, of course, and the horror of the atomic bomb. From that moment, really, the very word nuclear was tainted. It meant something that was going to kill you, in the form of lethal radiation that you can’t see. By the time of the “No Nukes” protests of the ’70s, to be “anti-nuclear” was to conflate nuclear weapons and nuclear power into a single category of scientific evil, a point of view whipped up, over the years, into a doctrinaire frenzy of righteous fear and loathing by anti-nuclear activists like Dr. Helen Caldicott and reinforced by movies like The China Syndrome and even, in its benign satirical way, The Simpsons.

Stone, a lifelong environmental lefty himself, unravels that thinking. The film’s incredibly articulate — and deeply progressive — spokemen and women explain the nuts and bolts of why nuclear power, manufactured with the sophisticated breeder reactors that are available today, is fundamentally clean, efficient, and, yes, safe. As Richard Rhodes puts it in the movie: “To be anti-nuclear is basically to be in favor of burning fossil fuels.” Pandora’s Promise makes a powerful case that in an age when former Third World countries, striving for modernization, are beginning to consume energy in much vaster amounts (and why shouldn’t they have the right to do so?), none of the alternative energy sources that are commonly talked about by environmentalists (wind, solar, etc.) can begin to fill the planet’s energy needs. Only nuclear energy can. That’s why France, faced with its own energy crisis several decades ago, went nuclear. (Eighty percent of France’s energy is now generated by nuclear power plants.)

Ah, you say, but what about Three Mile Island, Chernobyl, and Fukushima? The ultimate issue raised by nuclear power — the one that, according to conventional progressive thinking, stops the pro-nuclear argument right in its tracks — is, of course, the issue of safety. And the very names of those three locales cast a dark mythological shadow. You hear them and think: Meltdown. Radiation poisoning.Death. Disaster. But this is where, as a society, we desperately need more filmmakers like Robert Stone. Carefully, piece by piece, without hysteria and without dogma, he looks at the evidence of what actually happened during those three infamous catastrophes: the reality of the damage, and the reality of the aftermath. The results, if you truly listen to them, are almost spectacularly counterintuitive. They won’t leave you shaken. They will begin to shake you out of your old tired ways of thinking.

The most startling argument mounted by Pandora’s Promise is that the rise of nuclear power is not merely a good thing, but probably inevitable, because it is, in fact, the only way that we can power the planet and save it at the same time. In what has to be the ultimate liberal-documentary irony, Stone demonstrates that the dire threat of global warming all but demands nuclear power as the key to its solution. Without it, the debate will go on, but carbon dioxide will continue to fill the atmosphere, and liberals everywhere, caught up in reflexive modes of environmental “activism” that are now not just complacent but perilously out-of-date, will continue to let their anxieties trump reality.